In a wireless system, it may be advantageous to have an accurate measure of the noise level at the receiver, so that various corrections may be made for the noise. The noise level may be measured, for example, by the average power of a sequence of consecutive samples of the channel impulse response, wherein the selected samples are believed not to contain signal components. The channel impulse response may be extracted by an inverse fast Fourier transform (IFFT) of a properly processed reference signal or pilot sequence available at the receiver.
For example, in an orthogonal frequency-division multiplexing (OFDM) system such as 3GPP Long Term Evolution (LTE) or New Radio (NR), the demodulation reference signals (DMRS) may be used for the extraction of the channel impulse response. The DMRS may be transmitted in a plurality of resource elements, wherein a resource element may be a time-frequency unit in the OFDM grid (i.e., one OFDM symbol in the time domain and one subcarrier in the frequency domain). The signal in the DMRS resource elements may be descrambled by multiplying it by the complex conjugate of the known scrambling sequence; then, de-spreading may be performed, by multiplying the signal by a known orthogonal cover code in the time domain and/or frequency domain. The channel impulse response may be then calculated as the inverse fast Fourier transform (IFFT) of the de-spread descrambled signal. The noise may then be estimated using the calculated channel impulse response.
Using a predefined sequence of consecutive samples, of the inverse fast Fourier transform, however, may result in an inaccurate estimate if the sequence of samples used includes samples that are likely to contain a significant amount of signal energy. Although OFDM systems such as Long Term Evolution are mentioned herein as applications to which the present disclosure may be relevant, the relevance of this disclosure is not limited to such systems, and it may be relevant to any system in which, in a sequence of samples, all of which contain noise, and some of which also contain signal, the influence of samples containing signal is to be reduced in forming an estimate of the noise power.